Add Mjolnir documentation site

[DavidBakerEffendi]

Adds a complete Astro Starlight documentation site for Mjolnir, consolidates the oversized README into focused docs, and gives the project a distinct purple/thunder terminal visual identity.

Key Changes:

• Adds the branded responsive homepage, system-aware light/dark theme, and deterministic ASCII hammer social card.
• Documents installation, the 10-minute evaluation, Council behavior, adapters, permissions, sessions/worktrees, headless/remote use, data boundaries, CLI, and licensing.
• Adds a checked evaluation fixture and GitHub Pages deployment limited to version tags and manual runs.
• Refocuses README and CONTRIBUTING guidance around the documentation site.

Validation:

• npm run check
• npm run social-card
• npm run build (18 pages; 833 internal links)
• python3 -m unittest discover -s fixtures/ten-minute-evaluation -p 'test_*.py'
• git diff --check

Touch Points:

• docs/
• README.md
• CONTRIBUTING.md
• .github/workflows/docs.yml